North Korea on Sunday warned of a "new type of nuclear test" to further strengthen its nuclear deterrence, protesting the UN Security Council's denunciation of Pyongyang's firing of ballistic missiles last week.
Last Thursday, the UNSC issued its verbal condemnation, saying Pyongyang violated its resolutions banning any launches using ballistic missile technology. It also said it would continue to consult on an "appropriate response."
"(We) will not rule out carrying out a new type of nuclear test to further bolster nuclear deterrence," the North's Foreign Ministry said in a statement carried by the North's official Korean Central News Agency.
Earlier this month, the North threatened to "show off" its nuclear deterrence capability. But it is the first time for it to mention the possibility of a fourth nuclear test, which would further sophisticate its ability to build nuclear warheads.
Analysts said that the "new type" of nuclear test appeared to mean a uranium-based atomic test or a test of a boosted fission weapon, the development of which is a pre-step toward the development of a hydrogen bomb.
Some observers say the North could also consider launching scud or Rodong missiles carrying atomic weapons to show to the world that it has succeeded in miniaturizing nuclear warheads.
The warning of another nuclear test came on the heels of President Park Geun-hye's announcement in Germany of her three-point agenda to lay the groundwork for reunification. The agenda includes expanding humanitarian aid to the North and bilateral exchanges.
Sunday's statement, in particular, berated the US for conducting joint military drills with South Korea, while arguing that its missile launches proceeded as part of a "self defence-oriented" exercise.
"The US is relentlessly carrying out what it calls 'annual' exercises for a nuclear war by mobilizing all nuclear strike means with a view to occupy Pyongyang," the statement said, claiming that it would do a variety of its own drills to respond to the allied exercises.
"Should there be any catastrophe no one wants to see on the peninsula, the US would be held responsible wholly for that," it said.
The North fired two midrange Rodong ballistic missiles from mobile launchers into the East Sea last Wednesday in an apparent show of displeasure with the increasing pressure on it to denuclearize.
The launches came just as the leaders of South Korea, the US and Japan reaffirmed their opposition to Pyongyang's possession of nuclear arms.
Experts said that through the warning of a nuclear test, Pyongyang aims to demonstrate its resistance against outside pressure and continue to keep its nuclear programme as a principal guarantor of its security.
Seoul officials believe the North is "technically ready" to conduct another nuclear test, and that the timing of another test would be based upon the reclusive regime's political calculations.
Observers said that for now, Pyongyang would not be inclined toward another test given strong opposition from its major patron and ally China, which has prioritized stability on the peninsula. For the North, stable relations with the outside world are critical to secure economic aid to help rebuild its moribund economy.